linux-kernel - Re: [PATCH V3] LoongArch: Provide kernel fpu functions

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Date:   Mon, 6 Mar 2023 20:49:19 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     maobibo <maobibo@...ngson.cn>
Cc:     Huacai Chen <chenhuacai@...ngson.cn>,
        Arnd Bergmann <arnd@...db.de>, loongarch@...ts.linux.dev,
        linux-arch@...r.kernel.org, Xuefeng Li <lixuefeng@...ngson.cn>,
        Guo Ren <guoren@...nel.org>, Xuerui Wang <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        linux-kernel@...r.kernel.org, loongson-kernel@...ts.loongnix.cn
Subject: Re: [PATCH V3] LoongArch: Provide kernel fpu functions

On Mon, Mar 6, 2023 at 8:03 PM maobibo <maobibo@...ngson.cn> wrote:
>
>
>
> 在 2023/3/6 17:59, Huacai Chen 写道:
> > Provide kernel_fpu_begin()/kernel_fpu_end() to allow the kernel itself
> > to use fpu. They can be used by some other kernel components, e.g., the
> > AMDGPU graphic driver for DCN.
> Since kernel is compiled with -msoft-float, I guess hw fpu will not be
> used in kernel by present:). However it is deserved to try.
> >
> > Reported-by: WANG Xuerui <kernel@...0n.name>
> > Tested-by: WANG Xuerui <kernel@...0n.name>
> > Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
> > ---
> > V2: Use non-GPL exports and update commit messages.
> > V3: Add spaces for coding style.
> >
> >  arch/loongarch/include/asm/fpu.h |  3 +++
> >  arch/loongarch/kernel/Makefile   |  2 +-
> >  arch/loongarch/kernel/kfpu.c     | 41 ++++++++++++++++++++++++++++++++
> >  3 files changed, 45 insertions(+), 1 deletion(-)
> >  create mode 100644 arch/loongarch/kernel/kfpu.c
> >
> > diff --git a/arch/loongarch/include/asm/fpu.h b/arch/loongarch/include/asm/fpu.h
> > index 358b254d9c1d..192f8e35d912 100644
> > --- a/arch/loongarch/include/asm/fpu.h
> > +++ b/arch/loongarch/include/asm/fpu.h
> > @@ -21,6 +21,9 @@
> >
> >  struct sigcontext;
> >
> > +extern void kernel_fpu_begin(void);
> > +extern void kernel_fpu_end(void);
> > +
> >  extern void _init_fpu(unsigned int);
> >  extern void _save_fp(struct loongarch_fpu *);
> >  extern void _restore_fp(struct loongarch_fpu *);
> > diff --git a/arch/loongarch/kernel/Makefile b/arch/loongarch/kernel/Makefile
> > index 78d4e3384305..9a72d91cd104 100644
> > --- a/arch/loongarch/kernel/Makefile
> > +++ b/arch/loongarch/kernel/Makefile
> > @@ -13,7 +13,7 @@ obj-y               += head.o cpu-probe.o cacheinfo.o env.o setup.o entry.o genex.o \
> >  obj-$(CONFIG_ACPI)           += acpi.o
> >  obj-$(CONFIG_EFI)            += efi.o
> >
> > -obj-$(CONFIG_CPU_HAS_FPU)    += fpu.o
> > +obj-$(CONFIG_CPU_HAS_FPU)    += fpu.o kfpu.o
> >
> >  obj-$(CONFIG_ARCH_STRICT_ALIGN)      += unaligned.o
> >
> > diff --git a/arch/loongarch/kernel/kfpu.c b/arch/loongarch/kernel/kfpu.c
> > new file mode 100644
> > index 000000000000..cd2a18fecdcc
> > --- /dev/null
> > +++ b/arch/loongarch/kernel/kfpu.c
> > @@ -0,0 +1,41 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/*
> > + * Copyright (C) 2023 Loongson Technology Corporation Limited
> > + */
> > +
> > +#include <linux/cpu.h>
> > +#include <linux/init.h>
> > +#include <asm/fpu.h>
> > +#include <asm/smp.h>
> > +
> > +static DEFINE_PER_CPU(bool, in_kernel_fpu);
> > +
> > +void kernel_fpu_begin(void)
> > +{
> > +     if (this_cpu_read(in_kernel_fpu))
> > +             return;
> > +
> > +     preempt_disable();
> > +     this_cpu_write(in_kernel_fpu, true);
> > +
> > +     if (!is_fpu_owner())
> > +             enable_fpu();
> > +     else
> > +             _save_fp(&current->thread.fpu);
> Do we need initialize fcsr rather than using random fcsr value
> of other processes? There may be fpu exception enabled by
> other tasks.
Emm, I think initialize fcsr to 0 is better here.

Huacai
>
> Regards
> Bibo,mao
> > +}
> > +EXPORT_SYMBOL(kernel_fpu_begin);
> > +
> > +void kernel_fpu_end(void)
> > +{
> > +     if (!this_cpu_read(in_kernel_fpu))
> > +             return;
> > +
> > +     if (!is_fpu_owner())
> > +             disable_fpu();
> > +     else
> > +             _restore_fp(&current->thread.fpu);
> > +
> > +     this_cpu_write(in_kernel_fpu, false);
> > +     preempt_enable();
> > +}
> > +EXPORT_SYMBOL(kernel_fpu_end);
>
>